keys

Called in list context, returns a list consisting of all the keys of the
named hash, or in Perl 5.12 or later only, the indices of an array. Perl
releases prior to 5.12 will produce a syntax error if you try to use an
array argument. In scalar context, returns the number of keys or indices.

Hash entries are returned in an apparently random order. The actual random
order is specific to a given hash; the exact same series of operations
on two hashes may result in a different order for each hash. Any insertion
into the hash may change the order, as will any deletion, with the exception
that the most recent key returned by each or
keys may be deleted without changing the order. So
long as a given hash is unmodified you may rely on
keys, values and each to repeatedly return the same order
as each other. See Algorithmic Complexity Attacks in perlsec for
details on why hash order is randomized. Aside from the guarantees
provided here the exact details of Perl's hash algorithm and the hash
traversal order are subject to change in any release of Perl. Tied hashes
may behave differently to Perl's hashes with respect to changes in order on
insertion and deletion of items.

As a side effect, calling keys resets the internal
iterator of the HASH or ARRAY (see each) before
yielding the keys. In
particular, calling keys in void context resets the
iterator with no other overhead.

Used as an lvalue, keys allows you to increase the
number of hash buckets
allocated for the given hash. This can gain you a measure of efficiency if
you know the hash is going to get big. (This is similar to pre-extending
an array by assigning a larger number to $#array.) If you say

then %hash
will have at least 200 buckets allocated for it--256 of them,
in fact, since it rounds up to the next power of two. These
buckets will be retained even if you do %hash = ()
, use undef%hash
if you want to free the storage while %hash
is still in scope.
You can't shrink the number of buckets allocated for the hash using
keys in this way (but you needn't worry about doing
this by accident, as trying has no effect). keys@array
in an lvalue
context is a syntax error.

Starting with Perl 5.14, an experimental feature allowed
keys to take a scalar expression. This experiment has
been deemed unsuccessful, and was removed as of Perl 5.24.

To avoid confusing would-be users of your code who are running earlier
versions of Perl with mysterious syntax errors, put this sort of thing at
the top of your file to signal that your code will work only on Perls of
a recent vintage: